In the United States a standard has been proposed for digitally encoded high definition television signals. This standard is essentially the same as the MPEG-2 standard, proposed by the Moving Picture Experts Group (MPEG) of the International Organization for Standardization (ISO). This standard is described in a draft internal standard (DIS) publication entitled "Information Technology--Generic Coding of Moving Pictures and Associated Audio, Recommendation H.626" ISO/IEC 13818-2 DIS, 3/94 which is available from the ISO and which is hereby incorporated by reference for its teaching on the MPEG-2 digital video coding standard.
The MPEG-2 standard is actually several different standards. In MPEG-2 several different profiles are defined, each corresponding to a different level of complexity of the encoded image. For each profile, different levels are defined, each level corresponding to a different image resolution. One of the MPEG-2 standards, known as Main Profile, Main Level is intended for coding video signals conforming to existing television standards (i.e., NTSC and PAL). Another standard, known as Main Profile, High Level is intended for coding high-definition television images. Images encoded according to the Main Profile, High Level standard may have as many as 1,152 active lines per image frame and 1,920 pixels per line.
The Main Profile, Main Level standard, on the other hand, defines a maximum picture size of 720 pixels per line and 567 lines per frame. At a frame rate of 30 frames per second, signals encoded according to this standard have a data rate of 13,063,680 pixels per second. By contrast, images encoded according to the Main Profile, High Level standard have a maximum data rate of 1,152*1,920*30 or 66,355,200 pixels per second. This data rate is more than five times the data rate of image data encoded according to the Main Profile Main Level standard. The standard proposed for HDTV encoding in the United States is a subset of this standard, having as many as 1,080 lines per frame, 1,920 pixels per line and a maximum frame rate, for this frame size, of 30 frames per second. The maximum data rate for this proposed standard is still far greater than the maximum data rate for the Main Profile, Main Level standard.
Because of this range of data rates, it is desirable to have a flexible decoder which is able to decode Main Profile, High Level data as well as Main Profile, Main Level data. Since these formats may be used in widely different applications, however, it may also be desirable to have less expensive decoders which operate on several different compression formats but which may not be able to decode formats at the higher levels.
The MPEG-2 standard defines a complex syntax which contains a mixture of data and control information. Some of this control information is used to enable the signals having several different formats to be covered by the standard. Under the MPEG-2 standard, both compressed video and audio may be transmitted. While these types of information are separately encoded and decoded, they are combined in the MPEG-2 standard under the transport layer. The transport layer defines system parameters which are used to synchronize the audio and video streams and provides a structure for transferring both types of information. The focus in this application is on the encoded video information.
Even this video information, however, may exist in several different formats. These formats define images, having differing numbers of picture elements (pixels) per line, differing numbers of lines per frame or field and differing numbers of frames or fields per second. In addition, the basic syntax of the MPEG-2 Main Profile video data defines a compressed bit stream representing a sequence of images in six layers, the Sequence layer, the Group of Pictures layer, the Picture layer, the Slice layer and the Macroblock layer. Each of these layers is introduced with control information. Finally, other control information, also known as side information, (e.g. frame type, macroblock pattern, image motion vectors, coefficient zigzag patterns and dequantization information) are interspersed throughout the coded bit stream.
To effectively receive the digital images, a decoder must recognize the control portions in both the transport layer and in the layers containing the encoded video information, extract the necessary control information, and use the extracted data to process the video signal information. In addition, the decoder processes input data at varying rates but receives the data at a constant rate. This data, when processed, is stored and used to decode other data, at least some of which has been encoded using motion compensated encoding techniques. In addition, the data is transformed from a block-oriented format, used to transmit and process the data, into a raster scan format that is used to display the data. In prior art decoding systems, these processing steps are performed using separate memories. In many cases, these memories are relatively large, require dedicated supporting hardware which adds to the cost and complexity of the total system. Because these systems use several memories, it is difficult to achieve cost reductions for scaled down versions which process formats having lower data rates.